After the Messenger of Allah (peace be upon him) started to like solitude and staying by himself, he often frequented Nur mountain, which is a mountain north of Mecca, staying there and worshipping Allah (Glorified is He) for many days and nights. The Messenger of Allah (peace be upon him) used to stay by himself in Hira cave at the top of the mountain, worshipping Allah (Glorified is He) and contemplating about the Universe and about the life of the people around him as well as the oppression, misguidance and bad behavior which had spread in the Meccan society. Because of this seclusion in worshipping Allah, now and then Allah (Glorified is He) sent him divine inspiration that lightened his heart and mind with sacred heavenly guidance.
Portents of prophethood appeared now and then to gradually prepare the Messenger of Allah’s soul to receive divine secrets (i.e. prophethood); Allah did not send him as a prophet or choose him for this great mission of receiving the Message until He (Glorified is He) had prepared him for such a noble status of receiving divine revelation and being sent to guide and direct the people.
Therefore, now and then the Messenger of Allah (peace be upon him) used to see light and hear sounds, although he did not see anyone near him! The Messenger of Allah (peace be upon him) did not pass by a stone or tree but they greeted him; he used to hear them say, "Peace be upon you, Oh Messenger of Allah."
But whenever he turned around, he did not see anyone. The Messenger of Allah (peace be upon him) told us about this, saying, ««I know a stone in Mecca which used to greet me with peace before I was sent as a prophet. And I still know where it is now» [related by Ahmed].
This state continued, with him hearing sounds and seeing light without seeing any person, until he began to fear he was going insane. The Messenger of Allah (peace be upon him) mentioned this to his wife, Khadijah (may Allah be pleased with her), saying «I see light and hear sounds, and I fear that I may be going insane» [related by Ahmed]. Consequently, Khadijah gently calmed him, so he was no longer worried about these matters.
Then Khadijah went to her cousin, who was an elderly priest (Waraqah ibn Nawfil), and told him what had happened to her husband. He told her, «If what you say is true, this is the same angel (i.e. Gibrael) that came to Moses. If he is sent as a Messenger when I am still alive, I will fight for him, support him, and believe in him» [related by Ahmed]. That Christian priest noticed events and expected something eventful would happen...the situation on the earth at that time was very bad; the people were immersed in great misguidance and they needed a saviour to take man, who had become despicable, by his hand and direct him towards his Creator to what is of benefit for them and what guides them, after there had been no divine revelation to mankind for more than 600 years.